In a video display system, the precision and the stability of the horizontal synchronization signal, abbreviated as Hsync, are the most important factors affecting the reliability or quality of the overall video display system.
Typically, a switching power supply (SPS) system is used to provide a required voltage to a video display system. From previous experience, the Hsync signal fluctuates unexpectedly due to the interference of the output of the switching power supply system.
In order to prevent the output of the switching power supply system from interfering the video display system, the conventional arts employ several different approaches to synchronize the operating clock of the switching power supply system with the Hsync signal.
As shown in FIG. 1, the first one of the conventional approaches, a horizontal synchronization signal 131 is retrieved from the fly-back transformer 13 to synchronize the triangle-wave generation circuit 14 which forces the operating clock 151 output from the oscillator 15 to be synchronized with the synchronization signal 131. Therefore, the operation within the primary circuit 11 and the outputs therefrom are all synchronized with the horizontal synchronization signal 131. By this approach, the reliable operation of the video display system is achieved. However, since the oscillator 15 and the primary circuit 11 falls within the high voltage primary side, double-insulation wire must be used for the signal line of Hsync signal 131 to meet the requirement of various safety standards. If double-insulation wire is not used, the high voltage of the primary side may be transmitted to the secondary side exposing the user of the video display system to the danger of electrical shock. This approach has higher costs due to the requirement of the double-insulation wire for Hsync signal 131. The block 12 is a horizontal circuit of the secondary side.
As shown in FIG. 2, the second of the conventional approaches, a horizontal synchronization signal 231 is retrieved from the fly-back transformer 23 and fed to an isolation transformer 24, and a synchronization signal 241 from the isolation transformer 24 is used to synchronize the triangle-wave generation circuit 26 which forces the operating clock 251 output from the oscillator 25 to be synchronized with the horizontal synchronization signal 231. Therefore, the operation within the primary circuit 21 and the outputs therefrom are all synchronized with the horizontal synchronization signal 231. By this approach, the reliable operation of the video display system is achieved, and due to the high voltage of the primary side is isolated from the secondary side by the isolation transformer 24, a wire of low-cost general type may be used for. the signal line 231. However, the cut-down of the wire costs for the signal line 231 is offset by the cost of an additional isolation transformer 24 for this approach. Therefore this approach is still not a better one. The block 22 is a horizontal circuit of the secondary side.
As shown in FIG. 3, the third of the conventional approaches, a horizontal synchronization signal 321 is retrieved from the horizontal circuit 32 and fed to an isolation photo-coupler 33, and a synchronization signal 331 output from the isolation photo-coupler 33 is used to synchronize the triangle-wave generation circuit 34 which forces the operating clock 351 output from the oscillator 35 to be synchronized with the horizontal synchronization signal 321. Therefore, the operation within the primary circuit 31 and the outputs therefrom are all synchronized with the horizontal synchronization signal 321. By this approach, the reliable operation of the video display system is achieved, and due to the high voltage of the primary side is isolated from the secondary side by the isolation photo-coupler 33, a wire of low-cost general type may be used for the signal line 321. However, the cut-down of the wire costs for the signal line 321 is offset by the cost of an additional isolation photo-coupler 33 for this approach. Therefore this approach, as the second one, is still not a better one. In a typical switching power supply system, a plurality of photo-couplers are used to transmit signals while isolating the high voltage of the primary side from the secondary side.